1. Field of the Invention
The present invention relates to a photo mask (also referred to as a reticle) which is used as a projection negative in the lithographic operation for the manufacture of semiconductor devices, and more particularly the invention relates to the improvements in and relating to a phase-shifted photo mask which includes phase shifting portions for changing the phase of a transmitted light.
2. Description of the Prior Art
Photo masks of the type heretofore used as a projection negative in the lithographic operation for the manufacture of semiconductor devices has been generally constructed so that a light shielding pattern of a metal such as chromium is formed on a glass substrate and the photo mask is composed of only a transparent portion and light shielding portions. Therefore, it is suffice for the inspection of defects in such photo mask to check the light shielding portions as to their arrangement and the presence of defects and undesired projections.
For instance, in the case of an inspection method for detecting the defects through the microscope observation of a photo mask by visible-light transmission illumination, it is only necessary to discriminate the light and shade in an image of the pattern formed on the photo mask. Also, when effecting an inspection by means of falling irradiation illumination, the reflected light from the light shielding portions is far greater in intensity than the reflected light from the transparent portion thus making it easy to effect the detection of defects on the basis of the light and shade of the image as in the case of the transmission illumination.
On the other hand, as photo masks designed to cope with the recent trend toward finer circuit patterns, there have been proposed various phase-shifted masks in which phase shifting portions for changing the phase of a transmitted light are provided at specified locations of a transparent portion thereby enhancing the contrast of a projected image. For instance, Japanese Patent Publication No. 62-50811 discloses a technique concerning a phase-shifted mask based on a spatial frequency modulating method.
However, the phase-shifted masks heretofore proposed have the following disadvantages with respect to the inspection of defects.
With the phase-shifted mask, it is necessary to inspect the arranged positions as well as the presence of damages and undesired projections with respect to not only the light shielding portions but also the phase shifting portions. Moreover, the thickness of the phase shifting portions must be inspected since the phase-shifting portions must be formed to a thickness which is determined by an equation d={.lambda./2.pi. (n-1)}.phi. (where .lambda. is the wavelength of exposure light and n is the refractive index of the phase shifting portions) with .phi. representing the desired phase difference.
However, the phase shifting portions are generally made of a plastic material such as PMMA (polymethyl methacrylate) or glass such as SiO.sub.2 or SOG (spin-on-glass)(or a quasi-glass material) so that they are transparent to light ranging from the ultraviolet region to the visible region and they are practically equal in optical characteristics with the substrate (generally made of a synthetic quartz). Therefore, it is extremely difficult to observe the phase shifting portions by microscope observation with visible light and no thickness inspecting method is known in the art.